J. Pielaszek

Hydrodechlorination of CCl2F2 (CFC-12) over Pd-Au/C catalysts

Abstract A series of carbon-supported palladium-gold (Pd-Au) catalysts prepared by direct redox reaction method and characterized by various techniques were investigated in the reaction of dichlorodifluoromethane (CFC-12) with dihydrogen. The selectivity towards difluoromethane (desired reaction product) was increased upon introducing gold to palladium, from ∼72 to ∼86%, at 180°C. Such a selectivity enhancement was not observed in our previous studies when Pd-Au/C catalysts prepared by incipient wetness impregnation showed inadequate extent of Pd-Au alloying. Conditions of preparation of Pd-Au/C catalysts by the direct redox reaction method are found to affect the amount of deposited metals and the degree of Pd-Au mixing. The latter factor is essential in determining hydrodehalogenation behavior of the catalysts.

Structural and magnetic properties of RFe2H5 hydrides (R=Y, Er)

The structural and magnetic properties of RFe 2 hydrides (R=Y, Er) synthesized under high hydrogen pressures of 10 kbar and 373 K, have been investigated. Both saturated RFe 2 H 5 hydrides absorb about 5 H/f.u. and crystallize in the same type of orthorhombic structure (Imm2 space group), with a=5.437(1) A, b=5.850(1) A and c=8.083(1) A for YFe 2 H 5 and a=5.424(1) A, b=5.793(1) A and c=8.009(1) A for ErFe 2 H 5 . The large hydrogen pressure, necessary to reach 5 H/f.u. can be attributed to a partial filling of all types of available interstitial sites. YFe 2 H 5 displays a weak ferromagnetic behavior, which is almost independent of the temperature. ErFe 2 H 5 shows also a weak magnetization at high temperature whereas below 20 K its magnetization sharply increases due to Er magnetic ordering.

Neopentane conversion over Pd/γ-Al2O3

X-ray diffraction showed that during high temperature reduction at 600°C, chlorine-free Pd/γ-Al2O3 undergoes partial transformation to a Pd-Al alloy, which confirms results of other studies [9]. This evolution appears to have a large effect on the catalytic behaviour in the reaction of neopentane with hydrogen: the selectivity towards isomerization increases from <20 up to ∼80%. At the same time, the activation energy drops from ∼ 60 to ∼ 22 kcal/mol. These changes can be reversed by oxidation at 500°C followed by reduction at 300°C. The presence of residual chlorine (ex-PdCl2 precursor) appears to inhibit the Pd induced reduction of Al2O3 leading to Pd-Al alloy formation.

Hydrodechlorination of CCl2F2 (CFC-12) by carbon- and MgF2-supported palladium and palladium-gold catalysts

Carbon- and MgF 2 -supported palladium catalysts exhibited comparable activity, selectivity pattern and stability in hydrodechlorination of CCl 2 F 2 . Selective hydrodehalogenation to CH 2 F 2 was a prevailing reaction. Introduction of gold to supported palladium catalysts produced different results, depending on whether Pd/C or Pd/MgF 2 was doped. In the case of Au addition to 2 wt% Pd/MgF 2 , the selectivity to CH 2 F 2 was considerably enhanced, from ∼72 to 86%, whereas an analogous modification of 1 wt% Pd/C caused only insignificant changes. TPR and XRD studies of the catalysts indicated considerable differences in the extent of the homogeneity of the Pd-Au bimetal: the degree of alloying was substantially higher for the former catalyst. This suggests that an intimate contact between Pd and Au is essential for improving the selectivity to CH 2 F 2 . XRD of used catalysts showed carbon incorporation into a palladium lattice. A majority of this carbon can be removed by a short H 2 purge at 200°C.

Transformation of Pd/SiO2 Catalysts During High Temperature Reduction

During reduction in dihydrogen at 873 K, a silica-supported palladium catalyst is initially converted to Pd4Si which further reacts with silicon-containing species to a Pd3Si phase. Such a stepwise phase change suggests that the mechanism of Pd silicide formation involves incorporation of silicon into a palladium phase, not vice versa, as was suggested in our previous work. This transformation has a great effect on catalytic properties of Pd–silica-containing systems in 2,2-dimethylpropane hydroconversion.

X-ray diffraction study of the palladium-carbon system

Abstract The Pd-C solid solution was studied by wide angle X-ray diffraction. The Debye-Waller parameter and lattice strains were estimated for both carbon free and carburized palladium. Nearly equal values of the mean displacement of about 0.093 A was found for palladium atoms in these two cases, a value of 0.17 A was estimated for the carbon atoms in the carburized sample. An increase of lattice strains was found after decarbonization. The location of carbon atoms in the octahedral voids was confirmed.

In situ EXAFS study of the alloy catalyst Pd-Co (50%/50%)/SiO2

In situ extended X-ray absorption fine structure (EXAFS) data for nanocrystalline bimetallic Pd-Co catalyst supported on silica, for Pd K and Co K absorption edges, are presented and analysed confirming previously developed atomistic model of segregation within the alloy metal particle. Supporting evidence is presented from dynamical X-ray diffraction (DXRD) data. The confirmed segregation model concerns energy minimised partial surface segregation of Pd in contrary to a complete segregation model that does not fit well the EXAFS data.

Potassium-Promoted Carbon-Based Iron Catalyst for Ammonia Synthesis. Effect of Fe Dispersion

Potassium-promoted iron catalysts supported on thermally modified, partly graphitized carbon were studied in the ammonia synthesis reaction. Iron nitrate was used as a precursor of the active phase and KOH or KNO3 were used as promoters. The kinetic studies of NH3 synthesis were carried out in a differential reactor under 63 bar and 90 bar pressure. Hydrogen chemisorption, X-ray diffraction and transmission electron microscopy experiments were performed to determine the dispersion of iron in the specimens. All the K+–Fe/C catalysts proved to be sensitive to ammonia, the NH3 partial pressure dependencies of their reaction rates being close to that of the commercial magnetite catalyst (KM I, H. Topsoe). The catalytic properties of the promoted Fe particles on carbon were shown to be dependent upon the iron dispersion, i.e. smaller particles exhibited higher turnover frequency in NH3 synthesis. It is suggested that either small Fe crystallites expose more highly active sites, e.g. C-7 (B-5) or the promotion of small crystallites by the alkali is more efficient.

Reaction of neopentane with hydrogen over Pd, Pt, Ir and Rh

The skeletal reaction of neopentane with dihydrogen has been studied over powders and (111) oriented films of palladium, platinum, iridium and rhodium. Various pretreatments of the metals have been used for changing the surface morphology (O2–H2 pretreatment, very high temperature reduction etc). The effect of O2–H2 pretreatment on the course of the reaction was most pronounced in case of palladium (selectivity change) and platinum (overall activity change). The catalytic behaviour of rhodium and iridium suffers only minor changes after these pretreatments: demethylation always predominates. In contrast to unsintered palladium, heavily annealed Pd samples should have a lower density of surface imperfections. This fact seems to be responsible for the negligible activity of sintered Pd surfaces in hydrogenolysis and, in effect, for the higher selectivity toward isomerization. XRD analysis of variously pretreated metal powders shows that the increase in isomerization selectivity may be attributed to the sintering of palladium. At ca. 200 °C iridium powder was only slightly selective toward isomerization (2–3 %). This selectivity is limited to the initial stage of the reaction only and to clean, but not carbided, surfaces of iridium. A carbided Ir surface produces unusually large amounts of ethane among the hydrogenolysis products. The comparison between catalytic behaviour of platinum and other Group VIII metals allows us to rationalize the results in terms of surface morphology and the reaction mechanism.

Ab initio test of the Warren-Averbach analysis on model palladium nanocrystals

Model powder diffraction patterns were calculated via the Debye formula from atom positions of a range of energy-relaxed closed-shell cubooctahedral clusters. The energy relaxation employed the Sutton–Chen potential scheme with parameters for palladium. The assumed cluster size distribution followed lognormal distribution of a crystallite volume centred with the diameter of 5 nm, as well as two bimodal lognormal distributions centred around 4 nm and 7 nm. These models allowed an in-depth analysis of the Warren–Averbach method of separating strain and size effects in a peak shape Fourier analysis. The atom-displacement distribution in the relaxed clusters could be directly computed, as well as the strain Fourier coefficients. The results showed that in the case of the unimodal size distribution, the method can still be successfully used for obtaining the column length distribution. However, the strain Fourier coefficients obtained from three reflections (002, 004 and 008) cannot be reliably estimated with the Warren–Averbach method. The primary cause is a non-Gaussian strain distribution and a shift of the diffraction maximum, inherent to the nanoparticles, differing for every constituent cluster in the size distribution. For the bimodal size distributions, the obtained column length distributions tend to be shifted towards the centres of the modes and are less sensitive to the larger size mode.